Comparison of Trapped Field Characteristic of Bulk Magnet System Using Various Type Refrigerators

We developed several type superconducting bulk magnets with the goal of their industrial application, and study to improve a magnetic field activated by pulsed field magnetization (PFM). When considering a practical use of bulk magnet, it is important problem to choose a suitable refrigerator to cool a bulk superconductor. This paper investigated trapped field characteristics when using several type refrigerators; one is a Stirling refrigerator with an ultimate temperature of 40 K at no-load condition and cooling capacity of 11 W at 77 K. The other is a dual-stage GM type refrigerator with the cooling ability of 13 K and 5 W at 20 K. Now, we developed a new bulk magnet system using a dual-stage GM refrigerator with the cooling ability of 12 K and 12 W at 20 K aiming to improve a trapped field by removing heat generated after a pulse application quickly. Using a GdBCO bulk with dimensions of 60 mm in diameter and 20 mm thick, magnetizing tests were carried out and the relationship between trapped field performance and cooling ability was investigated. Even the bulk magnet using Stirling refrigerator with relatively low cooling capacity could trap a high magnetic field of 3.0 T on the bulk surface and maximum total flux of 2.0 mWb. The trapped magnetic field of new bulk magnet system was improved greatly at low temperature; the maximum trapped field of 3.6 T and the maximum total flux of 2.9 mWb were achieved.